Devices for varying or changing the angular phase relation or timing between an engine camshaft and a crankshaft are well known, as may be seen by reference to U.S. Pat. Nos. 3,626,720 and 4,754,727 which are both assigned to the assignee of this patent and which are both incorporated herein by reference.
The Meachum U.S. Pat. No. 3,626,720 includes a helical ball spline mechanism for varying the phase relation in response to selective porting of engine oil pressure to axially displace a piston therein.
The Hampton U.S. Pat. No. 4,754,727 discloses an axially displaceable advancing plate drivingly interconnecting support and drive members via straight and angular lugs or splines. The advancing plate is also threadably mounted on a drum for axial displacement along the drum in response to relative rotation between the drum and plate. Relative rotation in one direction is provided by a spring reacting between the hub and drum and in the other direction by selective application of a frictional force for retarding rotation of the drum counter to the spring force.
In an internal combustion engine, optimization of valve opening and closing events are a function of various operating parameters such as engine speed and load. Whenever valve opening and closing events are fixed for all engine operating conditions, valve timing, by necessity, represents a design compromise which detracts from engine efficiency and all but a limited range of operating conditions. For this reason, various prior art control systems such as those described herein have been proposed which actively vary valve timing during engine operation.
The majority of successful prior art approaches, like the foregoing cited references, employ a linear actuator to effect a desired phase change which converts the linear movement of the actuator into a rotary movement of the camshaft relative to its drive pulley or gear. Such devices have certain inherent disadvantages which have prevented their being widely employed in mass production. They tend to be difficult to package in the space envelope normally provided. Furthermore, their mechanical complexity has exacerbated cost and reliability.
More recently, approaches have been proposed which enable continuous control over phase change through a large range of operation. Although continuous control is desirable in certain applications, in others, a simple two position control is deemed adequate.
On certain types of cam phasers, it is necessary to cause an element on the phaser to rotate relative to the body of the device for purposes of actuation. This rotation is termed "self-actuating" if due to the fact that it uses the small engine induced speed changes of the camshaft due to individual valve events and, to a lesser degree crankshaft speed changes due to individual cylinder firing events, and does not require any form of outside energy input. Although ratchet and pawl arrangements have been proposed to provide such self-actuation, they can suffer from certain shortcomings in that any commercially feasible ratchet tooth geometry is relatively large compared to the minute camshaft instantaneous angular position variances due to individual valve events, and thus such devices can be ineffective in producing enough useful travel to effect the desired phase change. Furthermore, inherent in any pawl/ratchet arrangement, is the point of operation in which the pawl momentarily looses positive contact with an adjacent ratchet tooth whereby uncontrolled slippage and loss of phase control between the camshaft and crankshaft is possible. Finally, pawl/ratchet arrangements tend to be slow to effect a phase change, requiring many engine revolutions.
Although the present invention is described in its preferred application within an internal combustion engine for controlling phase relationship between the engine crankshaft and camshaft, this description is by way of example only, other applications being deemed self evident in view of the present specification.